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Constant speed propeller blade pitch compared to fixed-pitch
I just transitioned to a complex airplane (Piper Arrow II). While I get how to use the Manifold Pressure / Propeller levers, I still find myself trying to piece together how all the components would function in a given situation.
One such situation is with the prop lever full-forward. If you back off the manifold pressure with the prop full-forward, the RPM will drop because there isn't enough air / fuel for the engine to turn at a the request rate. Think coming in to land (prop full toward and start reducing manifold pressure).
My question is: Does the propeller flatten out (relatively speaking) more than in a fixed-pitch propeller aircraft in this situation or is it the same net effect as operating a fixed-pitch propeller plane?
I've heard that putting the prop full-forward and then just using the throttle is the equivalent of operating a fixed-pitch propeller plane. How accurate is this statement?

I believe an Arrow operates the same as my Cardinal.
With low manifold pressure, there is not enough oil pressure to drive the prop into anything other than flattest configuration no matter where the prop control is at.
If you were take off with prop control not all the way in, then in theory the prop would start getting more twist as engine speed came up, and it would be like taking off in 4th gear or similar, when you want to start your acceleration in 1st gear and then shift up (think of how the RPMs bog down as you cycle the prop at 1700 to ensure the prop can twist).
So you start with the prop and controls all the way in to take off (balls to the wall - RPM, prop,  mixture, carb heat - or levers if so equipped).
Once you get to altitude, you can start bringing them back, equivalant to shifting up and taking your foot off the gas (and leaning the mixture for increased). RPMs are dialed down, prop control is dialed out to increase pitch.
Then when you come from altitude and prep for landing, you play the game of reducing RPM, decreasing pitch, reducing RPM, decreasing pitch, until eventually you are in the pattern, prop is all the way to support a go-around, and RPM controls all the power, same as taking off.

Yes.  The prop has a low pitch (fine/high rpm) and high pitch (coarse/low rpm) stop.  The low pitch stop sets the blades quite a bit finer than you would have with a fixed pitch propeller, which is usually set at a pitch that will give more or less redline RPM while climbing at wide open throttle.
If a constant speed prop failed and the blades went to the fine pitch stop, the engine would overspeed at anything close to full power and you would have to throttle well back to keep the prop at redline.
Within the prop governor, wherever you set the prop control just applies a spring preload on a flyweight mechanism that regulates oil pressure to the prop to drive the blades within the operating range.  It's a compact version of those spinny ball thingy governors you see on steam powered farm tractors, but with everything hidden inside the governor housing.
Like the steam tractor, the fly weights are driven off the engine. They operate an oil control valve that controls oil flow to the prop and are, obviously, rpm sensitive. The oil control valve has a null position that fixes the blades where they are, and moves one way to move the blades coarse, the other way to move them fine.  When the prop is at the target rpm, the flyweights position the valve at null and the blades are fixed, hydraulically locked there.  If the engine speeds up, the flyweights move out, operating the valve to drive the blades more coarse until the speed is back to the target setting, then back to null.  And vise versa.
If you set the rpm to max, the flyweight governor spring preload is reduced to a point where the control valve null point is at redline rpm.  If there is enough power being applied, the governor will adjust blade angle to regulate rpm at redline.  When you reduce power, the governor will move the blades finer and finer to keep the rpm setting until they reach the fine pitch stop.  Once at that point, if you reduce power further, the blades can't go any finer to keep the rpm up, and rpm starts to drop and you effectively, temporarily, have a fixed pitch prop set way too fine to be of any use as a fixed pitch prop.
The point at which rpm starts to drop because power is insufficient to keep rpm at redline, even with the blades as fine as they can go, is called "coming off the governor".

This is more of an operational question, but my understanding is that while coming in to land, your prop lever should be full-forward, but at the same time you've reduced power to the point where the RPMs actually are dropping ("coming off the governor" as you mentioned). So, all things being equal, in this configuration, 1700 RPM isn't really the equivalent of 1700 RPM in a fixed-pitch propeller plane because the blade is much more fine. Would this insinuate that you'd want to operate at a higher RPM with a constant speed prop since it is being less effective during this phase?

Just use whatever throttle you need to use to maintain the speed and glide path you are on and down worry about RPM, but it will be a bit higher than the same conditions with a fixed prop.  Don't know if you've noticed it, but the blades going flat(-ish) trying to keep RPM up is why you get a much stronger braking effect when you suddenly remove power with a CS prop.  There is a flip side to this but is a bit more advanced; if the engine fails, you want the prop to be as coarse as possible, at the high pitch stops preferably, since it's closer to being feathered. So you pull full low RPM asap.

If the engine fails, you lose oil pressure. So the prop should revert to same as the prop control being full forward.

If it's a counterweight prop (most small GA props) the counterweights move the blades coarse so when oil pressure is lost they will move to full coarse anyway if the prop is still rotating enough for the weights to work. Going to low rpm helps it get there faster and is less dependent on the counterweights.  If it's a hydramatic (no weights), the only forces acting on the blades are oil pressure on each direction and blade pitching moment tending to fine pitch.  If you get the blades close to coarse pitch with the prop cont, once the oil pressure is gone they will more or less stay put.

I haven't flown piston GA complex aircraft, but I can provide some perspective from the turboprops I have flown.  Because they have been full feathering and reversible propellers though, this perspective could differ from your experience:
1.)  Yes, they flatten out much more than a fixed pitch.  At idle/beta the prop basically turns into a huge rotating speedbrake.
2.)  I am guessing that they simply meant that you fly it with throttle only like a fixed pitch because you don't have to constantly think about or adjust prop RPM when changing power settings.  If that is the case, then I agree with the statement.  Otherwise I would ask for clarification from anyone making this claim.

Here are 2 references for operation of constant speed props:  Google: How a Constant Speed Propeller Works, and Flying Magazine Constant Speed Prop Basics July 1, 2014.
The blue propeller knob controls blade pitch.  At full forward, for takeoff, climb out, landing
(preparation for Go-Around), the blade is set at a lower pitch relative to the direction of rotation, taking finer "bites" of the air. This is just like the lower gear of your car, providing
lots of acceleration but poor fuel economy.  Viewed from the front, yes, you could say it was
"flatter", opposite of being "feathered" (blade turned in the direction the plane is going to reduce drag if prop is shut down).
"Putting the prop full forward and using the throttle is the equivalent of operating a fixed pitch plane", well, the real advantage of variable pitch is being able to cruise the plane in "overdrive" by throttling back (reducing manifold pressure), then opening the blade pitch (pulling prop lever back) so the prop takes bigger bites at lower rpm.
But in the stressful enviornment of landing, take-off, airport traffic patterns, etc, once your prop AOA is reduced (prop lever forward), you can think of it as a fixed pitch if it helps, as you are only using the throttle to control engine output.
Recommend reading those articles.


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Is it possible to determine a rotation speed (Vr) using other known V-Speeds?
Is it possible to come up with a good estimate of rotation speed using other known V-Speeds?
For example: if I know a plane has a Vs @ 55kt. and a Vy @ 75kt, is there a reasonable method to use to come up with a safe rotation speed?
This is a general question. I'm not looking for an exact rotation speed calculation (unless one actually exists, of course).

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Should I wait for Tower to provide runway exit instructions?
This is somewhat already answered here (Is it a flight violation to mishear ATC instructions for exiting the runway?), but this question is more geared toward taking action before hearing from Tower.
I fly out of a towered airport somewhat often. 99% of the time when they provide me with a runway exit (Taxiway B in this case), it's the same one.
Sometimes however, they do not provide me an exit instruction quickly enough despite the fact that I'm rolling up to where I should be exiting for Taxiway B. They might either be busy with other aircraft or simply don't call out an exit instruction. Sometimes I've just rolled past Taxiway B and other times I start heading my aircraft in that direction just to finally hear Tower give me instructions to exit via Bravo.
So, the question is, if I know I'm almost always going to be assigned a given taxiway, should I take it or should I always be waiting for Tower to assign one and thus just keep idling down the runway until I get an assignment from them?
This becomes even more important when flying into unfamiliar airports. Do you just keep rolling or do you take a taxiway when you know you can?
I will like to clarify, 100% of the time, I have received a taxiway instruction AFTER I've started to point the nose of my aircraft at a taxiway. I have never actually exited the runway without finally receiving the instruction.

Just musing: If eg exit A is prior to exit B and you have not received instructions by the time you reach A then, based on the answers given, you should leave by A. Yes?

Note: I am assuming you are asking about US regulations.
If the tower does not give you an instruction, you should not wait for one. Leave the runway at the next available taxiway and then wait for further instructions:

4−3−20. Exiting the Runway After Landing
The following procedures must be followed after landing and reaching taxi speed.
a. Exit the runway without delay at the first available taxiway or on a taxiway as instructed by ATC. Pilots must not exit the landing runway onto another runway unless authorized by ATC. At airports with an operating control tower, pilots should not stop or reverse course on the runway without first obtaining ATC approval.
b. Taxi clear of the runway unless otherwise directed by ATC. An aircraft is considered clear of the runway when all parts of the aircraft are past the runway edge and there are no restrictions to its continued movement beyond the runway holding position markings. In the absence of ATC instructions, the pilot is expected to taxi clear of the landing runway by taxiing beyond the runway holding position markings associated with the landing runway, even if that requires the aircraft to protrude into or cross another taxiway or ramp area. Once all parts of the aircraft have crossed the runway holding position markings, the pilot must hold unless further instructions have been issued by ATC.

(Aeronautical Information Manual, emphasis mine)

Regardless of regulations, it is common sense to vacate runway as soon as possible. Not to diss the sensibility of the asker  TWR may assign a later exit to avoid creating intersecting traffic on ground.

@Bianfable - Good answer.  I would just add that the runway hold short markings support this, and mirror the convention of highway centerline markings:  i.e. if there is a solid line and a dashed line you may always cross from the dashed side to the solid side, but may never cross from the solid line side to the dashed side.  (without clearance...)  Feel free to add this detail to your answer and I will delete the comment.

A clarification that may be worth exploring: Reading between the lines of current answer implies- If Control fails to clearly communicate alternative instructions prior to the pilot exiting at the first safe opportunity, and this causes issues, then that is technically Control's problem rather than specifically the pilot's problem?

@TheLuckless Absolutely, the controller is under no obligation to give any instructions, but they have to deal with it, if this causes any issues. An exception would be if a NOTAM has been published that prohibits exiting via a specific taxiway (e.g. because of construction work). It is the pilot's duty to check all NOTAMs before and not to exit via this taxiway.

Just musing: If eg exit A is prior to exit B and he has not received instructions by he reaches A then, based on your answer, he should leave by A. Yes?

@RussellMcMahon Yes, if you are already slowed down to taxi speed, you must use exit A in this case. If you know that exit B is better you could delay slowing down to taxi speed until you are at B, but make sure not to block the runway unnecessarily long.

So Ryan Air always takes exist A :-). Only landing where I've felt strong cornering forces and strong deceleration at the same time (Dublin). Ryan AIr's southern wannabee clone, Virgin Australia provided about the strongest landing deceleration I've felt when we landed in Auckland from Oz yesterday.

In normal circumstances, you're not supposed to remain on a runway longer than you need for normal operational reasons, be it after landing, or before take-off.
Infact, at major/congested airports "Minimum RWY Occupancy Time" procedures are predetermined for both dep and arrival, so as to take the concept to it's limits. So, to minimize your time on the RWY is the conventional requirement, but the procedures to achieve this can be unconventional, for example,

(a)Crossing a stop bar without explicit spoken R/T permission (because, and onlybecause, the published procs for that particular RWY require it).

(b)Preferred exits published for certain runways, and these invariably designate exits other than the 1st. (where exit = taxiway)

Such variations to the original conventional procedures are normally very well documented with reminders given continuously via ATIS and other modes of comm so that pilots, ATC and all players are on the same procedural page.
Despite the above discussion, in case a situation requires it, you can be given ATC instructions to remain on the RWY, or permission to do so at the pilot's request.
Then there's LAHSO - Land And Hold Short Ops where it is mandated to stop on your landing runway, short of a certain cross RWY that's being used simultaneously, and await ATC instructions or then follow published instructions. Such procs can warrant training and approvals. The landing pilot must inform ATC early on, up in the air if it's not feasible for him to LAHSO. It cannot be 'mandated' for all, but refusing a LAHSO could lead to delays as the controller now has to readjust traffic sequencing and invariably reroute some.
Regarding the expectation of ATC instructions, to exit the RWY - The end of a landing roll often signals a whole load of actions for the cockpit, that too at speeds of upto 60Kts on a 'High Speed' or 'Rapid Exit' taxi track, and ATC is not supposed to interject at that point except if there's urgency/distress/emergency.
There are variations but as std procedure do not loiter on the RWY longer than absolutely needed.

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Why was the Tu-104 so prone to navball-disabling electrical power failures?
Of the (rather numerous) accidents involving the Tu-104, several were either caused or aggravated by major electrical failures, which disabled the aircraft's navballs, which caused the pilots to become spatially disoriented, which caused (or contributed to) a loss of control, which caused the aircraft to crash:

SU4 (15 August 1958) - en route from Khabarovsk to Irkutsk, the aircraft climbed too high while attempting to avoid storms ahead; recovery from the resulting stall was rendered impossible by "...disorientation of the crew, from accompanied failure of the artificial horizons".
SU3932 (30 September 1973) - shortly after takeoff from Sverdlovsk-Koltsovo, "[t]he aircraft crashed due to incorrect indications by the main artificial horizon and the compass system, caused by a failure of the electrical supply, resulting in spatial disorientation of the pilots."
SU964 (13 October 1973) - while on approach to Moscow-Domodedovo, the aircraft banked excessively far to the left, entered a spin, and crashed; "[t]he investigation concluded that after the first right turn executed by the aircraft (in which the bank exceeded 40°), multiple navigation instruments including the compass and artificial horizon failed. Combined with the poor visibility at the airport, the crew lost spatial orientation and were unable to see any landmarks to determine their position. When the plane banked slightly to the right, the pilots corrected the right bank only to put the plane into a sharp left bank that reached 70°, causing the crash."1
SU2415 (28 November 1976) - when making a right turn during climbout from Moscow-Sheremetyevo, the aircraft continued to turn past the assigned heading, rolled onto its side, and dove into the ground; "...among the causes of the crash included failure of the artificial horizon, poor visibility from the cockpit, and lack of warnings indicating the failure of the artificial horizon."

What made the Tu-104 so prone to these catastrophic electrical failures?

1: With 122 fatalities, this is still the worst Tu-104 crash to date.

Seeing the inherent unreliability, very poor handling characteristics, and tendency to enter an irrecoverable stall without warning, is it any surprise there were electrical problems?

The hull loss rate does not seem awfully different from other aircraft of the era like the Comet and the 707. Of course that's both a different question and something based on data unadjusted for hostile action or issues unrelated to the type, but be wary of seeing trends in what is essentially noise.

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Which tail type is better in order to obtain higher speed?
If I want to design an aircraft that can fly with high speed while loaded, which tail type should I choose.
The mission is to design an aircraft that can move a certain distance with high speed while carrying certain load.

What does "high speed" mean? Supersonic? Transonic? Hypersonic? 200kts?

Every single aircraft can transfer some load some distance at some speed. Presumably.

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Where can I find more information about my grandfather's crashed B-18a bomber on Mt. Redoubt Alaska?
My father and I have chartered a plane for late August to search for the remains of my grandfather's B-18a bomber, which crashed in the summer of 1942 in Mt. Redoubt Alaska.
I am specifically interested in advice from pilots who have flow around Mt. Redoubt:
A Saturday Evening Post article mentions the accident site is at 7500 feet MSL, on the southwest side of the mountain.  The accident report OTH, says the southeast side.  Questions:

What is the terrain/glacier situation at this location?
Is landing possible at this location?
Has anyone seen an accident site at this location?

I've posted additional details and pertinent material in this blog post.
Also, I would think the USAAF would have more detailed records of the rescue, which again, is detailed in the Saturday evening post article.  Perhaps even a dispatch to the rescue crew with a specific location.  Does anyone know where I might find such records?
Update, July 21st, 2017 (One month till departure)
Since I've offered a bounty, I guess I should iterate what I've done so far:

I've posted this to an Alaska bush pilot's forum.  I was surprised to not get a better response from this.
I've ordered (and received) a full sized topo map of the area from mytopo.com. The detail is pretty darn good.
I've scoured over multiple satellite photos.  Note, the obvious photos on google maps/earth are pretty iffy.  I found the photos at zoom.earth to be better.

Is there a question here? Please don't make people go to another site to read a long post if you have a specific question. This site is not a discussion forum, we tend to need specific questions with factual answers.

Fair enough...will update.

Not sure what else I can do???  I agree that the original post was vague about what I was asking.  However, my update is pretty specific.  Also, I wrote up the blog so that I can provide a specific link in forums I wanted to ask questions on.  Please take this off "hold".

Both putting and removing from hold takes 5 votes, and we are not constantly monitoring, so it can take a while, please be patient. I've edited the post for you to bring it more in line with the site.

Thanks.  I tried to add links to the post article and the accident report, but needed a rep of 10 to add more than two links.  I've been using Stackoverflow for years, but just discovered this specific exchange.

@bclarkreston - if you put links in like http:// www.xyz.com (note the space in there), someone will come along and fix them up for you so they're actual links.

Is it possible, although unlikely, that he is alove and able to show you the spot?

Google Maps shows details of about a couple of meters in this area, have you tried to search using it?

Oh yes, I've poured over quite a bit number of maps.  The best I've found so far, I've had to purchase from [http://www.mytopo.com].  The Satellite images on google maps/earth are not that great.  I actually found better ones at https:// zoom.earth

For the location of the crash, I'd trust the accident report over a newspaper report any day of the week.

With eruptions in 1962, 1989/1990 (converting brand new 747 into a glider) and 2009, I think Mount Redoubt has covered the remains of the B-18A airframe in volcanic ash and fresh snow by now.

What is the terrain/glacier situation [on the southeast side of Mt Redoubt]?

The southeast side is all glacier, forming in the valleys formed by previous formative eruptions, which have deepened significantly due to glacier flow. These glaciers do move and churn, so unless the plane crashed on top of one of the prominent ridges then the position of the plane's remains is changed, and they are likely mixed/buried within the glacier.
It's been 75 years though, and while the glaciers sped up during eruptive periods (or melted, as on the north face during the 1989 eruption), the average annual glacier speed is 20 meters per year.  So the wreckage would have moved at least 1.5km down the mountain slope over time, buried under new snow (and occasional ash), and always moving down into the drift glacier.
These reports on Mt. Redoubt's glaciers should provide significant help in determining the location, thickness, and movement of each glacier present:
https://pubs.usgs.gov/wri/1997/4187/report.pdf
https://www.igsoc.org/journal/32/112/igs_journal_vol32_issue112_pg355-362.pdf
In a normal crash if you have the location to within 100 meters it can be found with a simple search, even decades later, even though the plane may have undergone environmental extremes.  In this situation, though, not only will the wreckage have been moved, but it will have been torn apart and spread over a much larger area than the original crash.

Is landing possible at this location?

You can land on glaciers in general, and these are no exception, but you can't land on the side of Mt. Redoubt any easier than any other mountain.  You may be able to land at the base of the mountain, but you'd have to decide which glacier flow to land on, and as explained below it's unlikely that you will find anything to help you decide which one.
The pilot will have to make the final decision, though, as conditions may vary, not just in the weather, but on the glacier itself.

Has anyone seen an accident site at this location?

Only the rescuers would have.  Once consumed by the snow and glacier over years it would have remained only as small parts of the whole.  Any pieces remaining visible to the eye would also act as solar collectors - they would melt the snow beneath them, sinking lower into the glacier, and then be covered by another fresh layer of snow.
I commend you and your father for your dedication to this aspect of your family history, and wish you luck!


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Why did the first Boeing B17 test flight crash on takeoff?
Wikipedia says that the control surfaces were locked.
What does this mean, and if the control surfaces cannot be moved, how did the aircraft get into the air in the first place?

The prototype aircraft had small "blocks" (more like wedges) which were used to prevent the control surfaces moving in the wind and potentially damaging the control surfaces, or even moving the aircraft on the ground. A bit like putting chocks under the wheels to stop the aircraft rolling.
These were not removed when the aircraft was taken on a test flight, so the control surfaces could not move. Essentially, they were "fixed" in an approximately neutral position. Imagine if you couldn't move your steering wheel in your car because someone had put wedges next to your front wheels to prevent them turning: that's basically the situation the pilots were dealing with, but unlike you they can't just hit the brakes and stop once the aircraft is in the air.
The Boeing 299 / B-17 are "taildragger" aircraft.
As you can see in this photograph which I won't link directly due to the size, the aircraft's nose is pointing quite sharply "up" when on the ground.
Here's a B-17, which isn't identical to the Type 299 but is close enough that it makes no difference. Hopefully this gives you the visual to see why the aircraft will lift off at a high enough speed.

When accelerating the nose being pointed slightly up gives a positive angle of attack for the wings. This means that once the aircraft gets above stall speed, it will naturally lift off into the air. Essentially, the aircraft is already pitched slightly up before any control surfaces are moved.
That also means that when the aircraft takes off, it will climb - at least initially. With the locked elevators, there is no way to change the pitch angle of the aircraft, and the aircraft will either pitch up or down naturally - there is almost no chance that the aircraft would be perfectly trimmed by accident.
In this case, the position the elevators were locked caused the aircraft to continue pitching up until it stalled. The crew had no way to correct this and the aircraft stalled out, rolled and crashed into the ground.

This has absolutely nothing to do with being a tail-dragger! As the aircraft picks speed and the wings start to generate lift, the aircraft will pitch down, since the centre of lift is behind the main wheels. If it is trimmed for low speed (nose up), it will remain nose up and lift off, but if it is trimmed for high speed (nose down), it will happily continue rolling on the main wheels with tail raised.

Similarly a tricycle gear aircraft if trimmed for low speed will raise the nose wheel (centre of lift ahead of main gear there) and lift off while if trimmed for high speed, it will happily continue rolling on all three wheels. In fact the standard procedure for A320 warns against lifting the nose prematurely and recommends making the take-off roll with forward pressure on the stick when heavy (wing-mounted engines shift the empty CoG forward, so heavy also means CoG at aft limit).

The A320 is affected somewhat by the fact the wings are under the engines - not an issue in the B-17. The B-17 doesn't raise it's tail much naturally like a lighter taildragger, it lifts off almost (but not quite) in a 3-point-landing stance https://www.youtube.com/watch?v=PgpWuxYLhLU&nohtml5=False

So the problem was that the aircraft was trimmed for speed just above stall speed so it lifted off on its own, climbed a bit in ground effect and stalled as it left the ground effect and slowed down in the upper part of the phugoid cycle. It would have happened if it had a nose wheel too.

Yes, low mounted engines cause a pitch-up moment when running. But while that makes the problem worse, it does not create it. If the trim speed is above stall (in ground effect) and within what the plane can reach on the ground, it will lift off without control input and whether it has tricycle or conventional gear does not matter.

If B-17 lifts off in almost 3-point attitude, it is because it corresponds to the appropriate angle of attack. But if you trimmed it nose-down, it would still raise the tail and continue rolling on the ground and not lift off. Whether it lifts off depends on the trimmed speed, not type of gear.

I think you're missing the fundamental point - most B-17's will take off naturally with a neutral stick position. That's why the aircraft took off and, more importantly, why the pilots didn't notice prior to takeoff... They would have no major need to touch the yoke (or is it a stick in the B-17?) perhaps I should make the answer slightly more B-17 specific in that regard

I admit there is one difference, but it is in when the crew could notice something is wrong. In tail-dragger the crew is waiting for the tail to raise before doing anything, so they won't realize it is going to lift off early before the wheels are off the ground. In a tricycle, the crew expects the aircraft to only start rotating on positive control input, so when it starts raising the nose early, they will try to keep it on the ground and thus get a chance to cut the engines before they gain altitude. Still, it does not make the situation exclusive to tail-draggers, merely more dangerous.

@JonStory The pre-takeoff checklist is a pretty good reason to touch the yoke before takeoff. :) That Flight Controls: Free and Clear part is pretty important.

No, I am not. B-17 will take off naturally with neutral stick position. That was contributing factor in the accident. What I am saying is that it is not a consequence of it being a tail-dragger, but a consequence of how it is trimmed and that a tricycle aircraft trimmed the same way would lift off naturally with neutral stick position as well while if B-17 was trimmed differently, it would not lift off with neutral stick position.

@reirab this accident was one of the main catalysts for the introduction (or at least increased use) of the pre-takeoff checklist.

And I even see a reason to trim a tail-dragger nose-up while tricycle nose-down. In both cases, you want to keep the third wheel on the ground as long as possible to maintain directional control—and prop engines cause significant yawing moment during take-off roll so you definitely don't want the tail off the ground too early in B-17. But is still makes being a tail-dragger only an indirect reason for being trimmed to lift off with neutral stick.

If you follow through the links to the one on Ployer Peter Hill the B-17 (Model 299) test pilot, it says this about his death:

On October 30, 1935, Ployer Peter Hill died as a result of injuries received from the crash of the Boeing experimental aircraft Model 299 at Wright Field. The crash occurred because the crew neglected to remove the devices intended to keep the control surfaces from moving when the plane was on the ground. This aircraft was the prototype of what would later become the famous B-17 Flying Fortress of World War II. Major Hill was buried in Newburyport, Massachusetts, on November 3, 1935.

So basically they forgot to remove the gust-lock devices. Yet another case for doing a thorough pre-flight check and control check before taking off.
I can't find any evidence or anything to suggest the aircraft ever left the ground, however its possible for the aircraft to become airborne just through the amount of lift generated at higher speeds. Many airplanes will fly even with slight nose-down attitudes. Its also possible that the gust locks still allowed enough elevator movement to get off the ground. The link that Federico provided seems to suggest that the elevator was in the up position and allowed the aircraft to climb into a stall, but still not completely clear since most tail-draggers require some down-elevator to get the tail up, then up elevator to climb.

also: Yet another case for doing a thorough pre-flight check and control check before taking off. pre-flight checks were not a thing at the time. and, to say it all, they became a thing because of this accident: http://www.thisdayinaviation.com/30-october-1935/ As a direct result of this accident, the “check list” was developed, now required in all aircraft.

@Federico I modified the answer to respond to how it can fly. I understand that check-lists and pre-flights weren't around then, I was just commenting on the need for them now because even military test pilots can forget to check things.

Worth noting that a tail-dragger aircraft like the B-17 will generally lift off naturally when accelerating due to the nose-up attitude. This source suggests that the aircraft did indeed lift off before stalling: http://code7700.com/mishap_boeing_model_299.html

Please don't click on this link unless you understand that it shows an actual aircraft crash in which people died.  This video is a particularly horrific example of what happens when the gust locks are not removed.

@JonStory, tail-draggers will often be trimmed to lift off naturally, because they are trimmed to keep the tail on the ground until they pick up enough speed for the rudder to be efficient. But the nose-up attitude is only reason indirectly by motivating that default trim setting. Any aircraft will assume the trimmed attitude before lifting off.

(The accident in the video was on an air strip in Gimli, the same place where a 767 once landed after gliding from 12.500 m without fuel.) / Following check list is inportant: http://news.aviation-safety.net/2015/09/09/ntsb-series-of-errors-by-flight-crew-caused-fatal-gulfstream-g-iv-crash/

To be more precise, it was this accident that caused WRITTEN checklists to become universal.  Pilots did preflight checks before this accident, but they relied on memory to tell them what to check, and this particular pilot had a brainfade on that particular day, with bad results.


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Why is angle of attack information not displayed in the cockpit?
Would not it be useful to have the angle of attack information displayed on the flight display? I know that we have shakers that alert pilot when the critical AOA is exceeded, but still we have cases of pilots stalling the aircraft (sometimes stalling one wing more than other).
I am sure designers have considered this but not chose to display AOA. I am curious what the reason could be.

of which aircraft/flight display are we talking about?

AoA detectors are relatively expensive in GA aircrafts. Many jet airliners have them.

It has been a recommendation from the BEA after AF447 disaster (p 205) to display the AoA: "It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle  of  attack  could  enable  crews  to  rapidly identify  the  aerodynamic  situation  of the aeroplane and take the actions that may be required."

Many aircraft do include them, because it's very useful.
For example a modern 737:

A Garmin G1000 (one of the more common General Aviation glass cockpits):

Specifically the part that looks a bit like this

And a more traditional analog/mechanical gauge:

The problem is that usually when someone stalls, it's because they've made a mistake - not because they can't see the AoA. Either they're disorientated, or they've made a mistake with airspeed or an obstacle and are exceeding the aircraft's performance envelope.
Typically, the lack of an AoA dial isn't the reason you stall.

How would this display indicates if a single wing is stalled?

It generally doesn't, it relates to the position of the pitot tube I believe

I would take a further guess that when an accident happens due to a stall, it's because either the sensors have malfunctioned or the pilot doesn't believe the indicators.

Would you please make the AoA indicators more clear on the glass indicators? For the 737, I believe it's the 270° circle and 5.0 reading, but I'm not sure. For the Garmin, I believe I (not a pilot) can identify most of the indicators, but none jump out and scream AoA to me. Hand drawn red circles would be most useful... ;) Thanks!

For the 737 it is indeed the 270 degree circle and 5.0 reading. For the Garmin it's that traffic-light-coloured set of lines to the right of the speed tape: the equivalent of this http://garmin.blogs.com/.a/6a00d83451bb7069e201b8d0737e3e970c-800wi - the segments light up relating to your AoA and equate to the green/yellow/read areas on the mechanical indicator

Ah, thank you! I thought those lines on the Garmin were glide slope indicators of some sort

Nope the glide slope on a Garmin appears just to the left of the altitude tape and looks much like the one you'd get on a 737 or A320 PFD

@FreeMan This may be a matter of the point of view, but I think it's better to think of an accident that happens due to a stall being because the pilot has improperly flown the airplane, not because of sensor failure. The sensors are there to warn the pilot that he's messing up, but they are not the cause.

This came up a lot regarding the crash of Air France 447. "If only the pilots had an AoA indicator, they would have had known they were in a nose-up stall and corrected." However, since the pitot tubes were jammed, causing faulty air speed readings, its likely the AoA indicator would likely have been incorrect as well.

@AFischbein The AOA sensors on AF447 worked properly throughout. They are vane-style sensors not dependent on the pitot tubes. They did fail, however, on XL flight 888t

Slightly off-topic, but I recently came across the "side string" which gives a direct reading of AoA.  Every gilder pilot is familiar with the yaw string, but in two years of learning to fly gliders, I'd never heard anyone mention the side string.

Now, in the ultralight / open-cockpit world they use a single piece of yarn mounted on a stick, which acts as a combination yaw and side string.

In every single type of glider I have flown, the change in noise and attitude were so clear near the stall, that I would have never paid attention to a side string like that.

It strongly depends on the aircraft we're talking about.
As an example, this is the Head Up Display of an F/A-18 "Hornet":

The number near the α is the angle of attack (AoA or α) measured in degrees.
I presume that only in certain conditions knowing the exact value of the AoA may turn out to be useful (when nearing the limits maybe...). Usually, exceeding the maximum AoA is a the result of a mistake, not the mistake itself.
Fly-By-Wire systems usually do not allow the pilots to exceed the max AoA (Airbuses' Alpha Floor for instance) during normal operations.

20 degrees! Ouch, this guy must be pulling some serious G's.

Oh, it says 1.7G?

Do I remember correctly that the "W" symbol indicates where the nose is pointed and the circle with the three lines coming out of it, left right and upwards (at the bottom of the image just to the right of center) indicated the computed actual path of the airplane? If so that's yet another way to get a rough idea of the AoA from this kind of HUD.

@romkyns: Note the speed: At Mach 0.36 the F-18 will pull 1.7g but not 7.6; this needs far more speed.

@ToddWilcox, you're right, the "W-like" symbol indicates were the nose is pointed, the cross just over it is the "gun cross" that indicates the initial trajectory of the bullets fired from the gun while the "airplane" symbol represent the direction of the velocity vector of the airplane.

@PeterKämpf, romkyns, IIRC the number 7.6 should be the maximum pulled 'g' during the flight.

As others have noted many aircraft do have AOA indicators. Military A/C tend to have them next to the HUD on the left side.
The advantage using AOA over stall speed is that the stall AOA is constant while the stall speed depends upon weight. Greater weights => lower stall speeds.
This difference will be important when there are large differences in A/C weights. If your F-18 has 4x2000lb bombs the stall speed is much different depending upon whether those bombs are dropped or not.
With your average cessna, the weight change of the A/C/ does not change the stall speed significantly. An AOA indicator does not provide as much value relative to its cost and weight.

And with your average Cessna (or Piper &c) you tend to fly  a lot more by "seat of the pants" than by instruments (or at least I do).

An F-18's stall angle is the same whether or not it's carrying those bombs.  What changes is how fast it has to fly in order to stay in the air without exceeding that angle.

You are correct. A typo on my part. As the other paragraphs indicated, it is th stall speed that changes and not the AOA. Thanks for pointing it out.


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Why should the leading edge be blunt on low-speed, subsonic airfoils?
I have read that is to help the flow remains attached, but I do not understand the physical principle.

Probably related: http://aviation.stackexchange.com/questions/24414/why-when-is-the-blunt-nose-better

Might not apply, though, since this is about an airfoil and not an aircraft nose.

When using an airfoil with a sharp leading edge in subsonic regime, you need to adapt the angle of attack so that the stagnation point occurs right on the sharp edge.
At every flight condition there should be an AoA which achieves this. Higher AoA, and the stagnation point is too low. To low AoA, and the stagnation point is too high. However flight conditions change frequently, so the required AoA changes dynamically as well. Since AoA also drives the lift and lift is the priority in a pilot's mind (he wants to keep flying after all), stagnation point will only occasionally match the leading edge.
During flight the AoA will often fail to match the stagnation point, let's say with too high AoA, separation would occur below the leading edge (on the "intrados").  This is the only good illustration I could find:

The stagnation point is at the tip of the only grey line that stops right on the black plate.
Basically all incoming flow between these leading edge and stagnation point  (unfortunately no line shows this on the picture) will follow this trajectory:

Incoming at high speed onto the airfoil, below leading edge, going towards the stagnation point, but aims just slightly higher up.
The stagnation point is the point of highest pressure, so the incoming flow slows down
Comes a point where the slowdown is sufficient to actually reverse the flow which now shoots up along the surface towards the leading edge, now driven by the pressure from stagnation point
Arriving at the leading edge from below the flow needs to do a U-turn around the leading edge to keep following the wing shape and flow towards the "extrados" and keep on with its life.

Therein lies the problem: when finally arriving at the leading edge, the air has non-zero speed (zero speed only occurs at stagnation point, of which there is only one), but must instantly reverse it. Physics says nope, and you get a large leading edge vortex to smooth the motion out. Unfortunately that is also called a flow separation bubble, which may became a stall.
This is:

bad for performance (drag)
bad for safety (stallable)
bad for comfort (vibration from the eddies)
bad for structure (vibration fatigue)

Why does it only happen in subsonic flight?
At supersonic speeds, information cannot travel back on the flow using the same pressure gradient trick, once the air is on the intrados/extrados, it stays there, and cannot back up the other way. So no 'adaptation' is possible, regardless of AoA.
Why use rounded leading edge to remedy this in subsonic?
With a rounded edge, the U-turn is not so sharp, it is actually manageable for the air. This means you'll not always have flow separation, so you're safe. The rounder, the easier larger the range of AoA stay "adapted". This animated gif shows really well how the stagnation point can move around the rounded tip thus avoiding separation:

Source: UNIVERSITY OF GENOA
FACULTY OF ENGINEERING
DEPARTMENT OF ENVIRONMENTAL ENGINEERING
IRROTATIONAL PLANE FLOWS OF AN INVISCID FLUID
TL;DR It's possible to fly subsonic with sharp leading edge, but less safe and lower performance. The key is AoA adaptation.

Welcome to Aviation.SE! I edited your answer to embed the image (and provided the source info), instead of having to request that we click the link to see it. That's the preferred method around here (unless there are copyright issues involved). You can click the 'edit' button to see how I did it so you can do that on your next answer!

Oh very good. I knew I should've added the source. For embedded image I thought I needed more priviledges to do it... Looks neat, thanks!  Also for the rest, I didn't have source but my own [supposed] knowledge but people here seem to add a lot of references. I should look out more...

References for information are always good, but there are lots and lots of unreferenced answers, don't worry about it.

The short answer is that a blunt leading edge tolerates a wider envelope of angle of attack than a sharp leading edge.  Making it even simpler to understand it makes the airplane more forgiving to fly than a sharp leading edge.
I'm certain someone will give a more thorough explanation consistent with what I have said here.  I just thought it would be useful to give a quick reply.

There is no requirement for a blunt leading edge. Some low speed subsonic airfoils, such as the Wainfan Facetmobile, have sharp leading edges. Airfoil design has a lot of tradeoffs, lift/drag, predictable stall characteristics, etc.


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Are crosswind landings performed using autoland or under manual control?
I believe many people here have seen at least one of Cargospotter's videos of crosswind landings at Düsseldorf (DUS/EDDL).
There are several of them. I picked this one:

I think the pilots land their aircraft manually in case of such heavy crosswinds.
But is that true? Or do they lean back and watch the scene as the autolanding system does its job?
This somehow related question suggests the plane is operated manually.
Are there any regulations as to when using autolanding and when to do it manually? I know this might be
opinion-based, but could autoland even handle these crosswind situations?

One reason why autoland systems haven't been developed with high crosswind capability is that autoland is needed most for low-visibility conditions, which usually coincide with calm or light winds.

@pericynthion Or with conditions in which landing isn't safe anyway, such as a strong thunderstorm.

It's not opinion based. The limits will be specified for each aircraft type and autopilot system installed. The company may also restrict this further. Rather than have a specific regulations it will defer to the type certification and manufacturers limits

The B777 is limited to a max crosswind component of 25 knots when using Autoland so it is most likely that it was a manual landing in the photo above.
The B777 has a "max demonstrated crosswind component" of 38 knots. Some companies will round this up to 40 knots for a company limit.
The "max demonstrated crosswind component" is not a limit. It just means the aircraft can land with that wind speed, in a crabbed configuration, without damage to the aircraft or landing gear.
Crosswind landings for certification are done in a crabbed configuration. Boeing has demonstrated landings for the B777 with a 38 knot crosswind, so that becomes the Demonstrated Crosswind Component.
If Boeing had tested with winds greater than 38 knots, AND it resulted in damage,  then the B777 would have a 38 knot Crosswind LIMIT.
Of course there was no damage at 38 knots and there is nothing preventing pilots from landing with crosswind components greater than 38 knots.
Since Boeing has not tested or demonstrated crabbed landings ABOVE 38 knots, we don’t know what the actual limit is when damage may occur due to side loading.
So when landing with a crosswind greater than 38 knots, it would be prudent to use a technique that would involve a slip and removal of any crab at touchdown so that excessive side loading is avoided.

May I ask if I understood this correctly: up to 25 knots the B777 pilots can use autoland, up to 38 knots they can/must land manually and above 38 knots they cannot land and must either wait or go elsewhere, right?

38 knots is really just a recommendation, not a limit.  Above 38 knots the pilot must remove the crab at the moment of touchdown, or use side slip.

But you better be very experienced with crosswind Landing technique and current as well.  Big gamble with 300+ lives aboard to casually do otherwise.

@MikeSowsun So, for the sake of argument, suppose a pilot does decide to land at higher speeds than 38 knot, how does the company later check whether or not damage indeed happened.

"Maximum demonstrated crosswind component" just means that's the highest they showed during certification testing; it doesn't mean that's the limit of what the plane can do safely. But 38kts is already quite high; they probably couldn't find any higher crosswinds during testing--outside of a tornado or hurricane, at least.

@ curious_cat     Landing with a crosswind above 38kts might cause damage if the pilot did not apply proper crosswind technique.  If there was damage, it would be discovered by maintenance personnel before the next flight since most wide body aircraft get a Pre Departure Inspection before each flight.

The definition in your last paragraph is simply wrong.  If it would cause damage when using the recommended landing technique, then they will impose a limitation, but max demonstrated is simply that:  the maximum crosswind component demonstrated during certification.

I have edited my answer to try and clear up any misunderstandings.


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How to reasonably determine minimum safe altitudes over city? (fixed wing)
How does one make a determination about what a minimum safe altitude is when over a city. Let's reference FAR 91.119, the two sections that talk about this:

(a) Anywhere. An altitude allowing, if a power unit fails, an
emergency landing without undue hazard to persons or property on the
surface.
(b) Over congested areas. Over any congested area of a city, town, or
settlement, or over any open air assembly of persons, an altitude of
1,000 feet above the highest obstacle within a horizontal radius of
2,000 feet of the aircraft.

"(b)" is black and white, no questions there, however "(a)" is much more subjective.
Let's say I'm over a city. For the sake of being specific, I'm not talking about downtown areas with skyscrapers, I'm referring to residential / commercial mixed areas, typical "suburbia". If the city is 15 miles in diameter and densely populated, I would think (erring on the side of caution) I would consider my glide distance when determining a MSA.
If I was directly over the center of this city, that would easily make my MSA 4,000ft AGL (using a Piper Cherokee 140 in this example). Clearly this would not be the determination most others would come to as many GA aircraft fly much lower than this over cities.
What elements come into determining an MSA in a scenario like the one I have described?

What would be considered a reasonable landing spot over a congested area?
What would be considered "undue hazard?"
What other elements come into justifying an MSA?

Are we to assume that congested areas do not offer suitable areas where emergency landings could not be accomplished without undue hazard to persons or property? I used to fly pipeline patrols where 91.119(b) was waived allowing us down to 500 AGL but (a) still applied to us. There are lots of parks, canals, empty lots, cleared right-of-ways, rail yards, and so forth that offer what I would consider to be suitable areas in, say, the congested areas of Chicago. I don't think I've ever flown over a congested area that did not offer some suitable emergency landing areas.

@JonathanWalters, you've nailed the exact type of discussions I'm looking for. I realize that there are justifications such as those you just mentioned that come into play when making a call on an appropriate MSA. I've updated my question to more specifically call out what I'm looking for.

Let's say I'm over a city. For the sake of being specific, I'm not talking about downtown areas with skyscrapers, I'm referring to residential / commercial mixed areas, typical "suburbia". If the city is 15 miles in diameter and densely populated, I would think (erring on the side of caution) I would consider my glide distance when determining a MSA.

Yes, that's what the FAA expects you to do per 91.119(a): Consider your aircraft's characteristics (glide ratio, landing distance required), your altitude, and the availability of landing areas. Select an altitude that lets you land (ditch, crash, whatever) with minimum risk to persons and property below you.

If I was directly over the center of this city, that would easily make my MSA 4,000ft AGL (using a Piper Cherokee 140 in this example).

That is not an unreasonable determination if the area you're overflying is heavily congested (streets are full of cars/people, no parking lots, fields, or parks to dead-stick into without hitting a person/car/building).
Note, however, that your emergency landing doesn't have to be a pretty one: Light planes routinely fly through the Hudson River SFRA at low altitudes - well below 4,000 feet - and the West Side Highway or Hudson River Park are not particularly hospitable landing sites (you would be hard-pressed to execute an emergency landing without endangering persons or property). That means your "emergency landing" strategy is going to be the same as US Airways Flight 1549: You're ditching in the river. You can safely do that in a Cherokee or a Cessna from 1000 feet or less most of the time since the surface of the river is not as congested as the streets and parks of Manhattan: You can turn out over the river and be clear of people and vessels.

To address your specific bullet point questions:

What would be considered a reasonable landing spot over a congested area?

One in which you can set your aircraft down without undue hazard to persons or property. (Yes, I realize that's very close to a circular definition.)
You don't have to make the landing pretty: you may damage the plane in the process, and you might damage some property or even kill a person on the ground in the process - sometimes you just can't foresee all the problems - but you should be looking out for landing areas that minimize the risk of injuring or killing people (priority #1) or damaging other stuff.

What would be considered "undue hazard?"

"Undue Hazard" is a legal concept, and it will be up to your lawyer and the FAA's lawyers to determine if the hazard posed by an emergency landing you make is "undue" (excessive, disproportionate to what operating at a "normal" altitude would have imposed).
For a common-sense definition look at your operation and imagine you were standing on the ground: If you would be asking yourself "What the heck is that pilot thinking flying so low?!" then there's a good chance your altitude is low enough that an emergency landing would constitute "undue hazard" to the folks on the ground.

What other elements come into justifying an MSA?

We already talked about the big ones: Aircraft performance & what kind of area you're flying over.
You also need to consider at least two other factors:

Pilot Skill
You know what the best short-field performance of your plane is from
the POH, but can YOU reliably execute a landing and hit those
numbers? Would you bet someone else's life on it?
Weather Conditions
What if your only landing area has a tailwind?
What if you have to fly into a headwind to reach your landing area - will you make it given your best glide speed and glide ratio?

And now apps like skydemon even have a circle around you that shows your glide distance available.


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Where can I find detailed steps on how to perform all ACS maneuvers?
All the maneuvers outlined in the Airman Certification Standards (slow flight, power on stalls, short field, etc.) have been taught by my instructor to me but on no occasion have I received reference material for each of these maneuvers. I find after long periods of not practicing these, it can sometimes become easy to interchange the order of operations of 2 steps in a given maneuver. I have my checkride scheduled and need to be able to reference these.
Where can I find the individual steps in each required maneuver?
Aircraft: PA-28-140 / Piper Cherokee
To be clear: I know that the ACS itself has bulleted items identifying required things the candidate must be able to demonstrate, but this still doesn't detail how to perform the maneuver.

The way that I did it was to spend 30 minutes on the ground with my instructor and write down the steps. I then turned them into checklists that I would follow during a flight. It would help in this question though if you mentioned what type of aircraft, some steps may be slightly different depending on aircraft.

If you are interested in what my checklists looked like, Here is a Google Docs Link, the formatting is a little off because of the paper I printed on, but you get the idea. It is for a Cessna 172-P. Please note that these were developed before the new ACS, so these go to full stall or stall horn.

Thanks @RonBeyer! This is definitely useful. That being said, I would still love to know what "standard" each task is compared against. If we're judged based on performing it correctly, I'd love to know where that document exists that details out these standards we are held to.

FAA-H-8083-3B Airplane Flying Handbook covers most of the things you need to demonstrate on the Practical Test. And its free on the FAA website.

Interesting question.  In the past, the preamble to the PTS showed reference material that can be used to throughout the PTS.
With the new Airman Certification Standard (ACS), those references are incorporated into each task.  For slow flight below it lists the references as FAA-H-8083-2, FAA-H-8083-3 and POH/AFM.
 Private Pilot Airplane ACS
Undoubtedly, it will list the Airplane Flying Handbook as a reference.  All the maneuvers and techniques are in that book.  It is generic and the POH takes preference if there are disagreements.
Airplane Flying Handbook (FAA-H-8083-3)
Risk Management Handbook (FAA-H-8082-2)


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Is it possible to change/repair an aircraft tire while in flight?
Consider the following scenario: A crew experiences a flat/burst tyre above V1.
Is it possible, in some type of aircraft, to change it while in flight?
This might include, but not limited to:

access the landing gear compartment
access hydraulic operated equipment
changing the damaged tyre

That would be an exciting James Bond move.

Are you assuming that the crew are trained and certified, that spare wheels are carried (you can't just change the tyre), that they have the necessary tools, test equipment and lifting gear in the landing gear bay and that the bay is pressurised and conditioned?  Are you also assuming that all of this would be safer than just carrying a burst tyre to your landing?

See also the Keys brothers endurance flight which involved routine engine maintenance while in flight.

I am just asking, not assuming, it is my doubt, only Jonathan Walters.

There was a time repairing an engine in flight was critically required (read Saint Exupéry). So the question is an opportunity for good answers.

@JonathanWalters I'd like to see them elaborate on what maintenance other than oil was one, but I can't find an article. Here is an article on a 172 that stayed aloft for almost 65 days (1559 hours) and it said that the oil filler/filter was installed inside the cabin so it could be changed in flight. Not much else was done that I can see though.

Accessing hydraulic lines & resevoirs is perfectly straightforward in plenty of aircraft -- the C-130 as one example.  With crew chiefs & their tools & spare parts on board, a temp repair on a hydraulic line would be possible. Highly unusual, but possible. Spare fluid is common enough to have on-board to replenish the resevoirs.
However, access to the tires... Nope.  I can't think of anything where you could do that, and it would have to be a designed-in capability. Typically, crew-accessible areas are pressurized, but where the wheels retract, aren't.  You have things like access to unpressurized bomb bays, and in unpressurized aircraft that's not a barrier, but still... If there are designs where the crew can access the tires in flight, I'm not picturing them.
As noted elsewhere, landing with a flat tire isn't typically a big deal.

Normally the answer would be no, but if you have

an airplane with a low minimum speed and plenty of gas,
a fast pickup truck,
a very, very long runway and
calm weather,

you can have someone get the needed tools, the spare wheel and have him climb on the back of a pickup truck. What follows will look like this:

Stuck landing gear on a PA-28 being freed from the ground (picture source)
This repair needed only a jerk on the gear to get it unstuck. Changing a wheel takes much longer and is practically impossible this way.

If perfectly executed, one could probably change a tire like that in about 5-6 passes. Getting the cotter pin out would be the hardest part; I wouldn't worry about re-installing it until after the excitement died down. And hopefully the brake backing plates haven't any safety wire. Nose wheel might be easier.

Changing a light aircraft tire from a moving truck would be a risky proposition, and not worth it since landing with a flat is not a particularly hazardous thing. A wrong move could end up with the person in the back of the truck getting chopped into pieces for no particularly good reason.

Is it just me, or does this seem irresponsibly dangerous? I would think the risk to lives would be much greater than landing with the one wheel up.

I think the best course of action would have been to land with the undercarriage up @FredLarson, rather than two down. I agree that trying to extend the gear was a risk not worth taking. When I first saw that video as a kid I was impressed, knowing what I do now I think it's dumb.

I think getting the tire off would be easier than getting the new tire on @JonathanWalters, and if you can't get the tire on it's worse than a flat. Trying to change a nose wheel would take a person perilously close to the prop.


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Aircraft fuselages being shipped on a boat
Earlier this morning, my wife and I were down by the docks in Southampton (UK), watching cruise-liners move about with our binoculars. Whilst panning around, we noticed two old aircraft bodies (/fuselages - are the terms interchangeable?) on a large trailer, looking like they were waiting to be loaded onto a cargo vessel.
They had the wings, tail & nose removed, but the cavities that this exposes had been covered by yellow maintenance covers. I can't imagine they are simply being scrapped if the innards were protected like this.
So, I imagine they are being shipped somewhere for spares, is this trade a regular thing, and we just don't normally see it?

They looked bloody ancient. Very tatty, old-fashioned paintwork. One of them had the number Z733UK. I don't know where I'd look that up.

The Southern Daily Echo posted some footage of one of these holding up the traffic a few days ago. Unfortunately, the clip stops before I could get a clear view of the aircraft, but somebody might recognise the livery. I imagine the aircraft had been scrapped, but somebody, somewhere found a use for the fuselages intact.

Nice footage, thanks!

I added a screenshot from the video. In the comments of the video there was mention of the wings and tail also being transported.

The shipper https://www.finnieheavyhaulage.com/ doesn't seem to think it's noteworthy.  Their webpage gallery and facebook page brag about other unusual loads, but not (yet) this one.

Can I just make a quick comment about this post and everything about it? I have tried to engage with a few Stack Exchange forums at various times throughout the last decade or so. In every case, within the first week, someone would inevitably either find something objectionable about a question, comment, or answer I posted, or they would lambaste me in a comment and point out 45 -50 ways I had failed to comply with their set of ridgid requirements. I'm not sure where that culture sprung from, but it turned be off every time. And then this post, no question, just coolness, and no insults!

Just a quick note. I've rolled back everyone's over-enthusiastic edits. The question was about these things being spotted by a quayside, not about them being hauled via trailer. Admittedly, the one leads to the other, but the title edit completely changed my question.

@Bignutz. Apart from the fact that the question was changed by (I'm sure) well-meaning folks, and I got an answer to a different question.

(I asked that initial question because an unpainted fuselage may look derelict while it is in fact brand new. Deleted the comment now that it's obvious it concerns a used fuselage)

@Sanchises. Thanks, that's no problem. It was the editing it into a different question I objected to.

You should transform the title in more precise question (this is a Q&A website) to ease navigation without opening each question. There may be other questions about transportation of aircraft parts by boat.

@ManuH Are there, though?

@SiHa not yet, but think about making your question future-proof.. and following the same conventions as other question on aviation.SE

Road and boat transport are usually 2 stages of the same journey.  Roads go everywhere, but have width and height limits that make transporting outsize items like aircraft difficult, disruptive and expensive. Boat transport has fewer size limits but canals don't go everywhere. So the usual route is start site->road->nearest accessible waterway->road->finish.

The aircraft you saw was N733UK (not Z733UK). It's a Boeing 737-300, most recently operated by European Aviation Group, and before that by easyJet  and bmibaby. It was stored at Bournemouth Airport in 2015 and subsequently dismantled.
This site suggests it's on its way to China to be used for training.
https://www.planespotters.net/airframe/boeing-737-300-n733uk-european-aviation-group/rovlz3

Road and boat transport of airliners is difficult and expensive. If possible, the last flight of an aircraft will end at a place where it can be dismantled.
Nevertheless, not all aircraft end up as scrap. Some have their final destination in a museum, or have a second live as a hotel, or restaurant, or some sort of decoration. These places can be far away from an airport and require transport over water and/or streets.
Retired aircraft are usually sold at auctions, and anybody may buy a 747
for their back yard (if it fits).
Next time you witness a transport like this, try to spot the registration number. If you have that number, it is very easy to look up the history and - possibly - the destination of that aircraft.
Examples:

Concorde F-BVFB travelling to the Technik Museum in Sinsheim (Germany):

Image source: Technik-Museum Sinsheim
Boeing 747 PH-BFB became a decoration for a hotel. It didn't have to travel far from Amsterdam airport, but it had to be pulled across a highway in the night.

Image source: Hart van Nederland

Not to mention multimode transportation (e.g. for the A380)

Thanks for the answer. Unfortunately, you answered a different question to the one I posted. Not your fault.

@siha  Hmm, could you please clarify what actually is your question?

Of course. I was simply asking if aircraft bodies are routinely shipped via boat, because it seems expensive, and not something I'd ever witnessed despite living at a major international port for over 30 years. Alas, someone saw fit to add a picture of it being shipped by road, and then someone edited the title (and the text, IIRC) and it became a different question. Which you answered. I'm aware that boat/sea is a subtle distinction, but I didn't ask about road shipment.

@SiHa I'm a sailor, 'on a boat' vs 'on the sea' doesn't sound like a subtle difference to me. These are two entirely different meanings. I was not aware of the discussion that developed under your question in the past 24h. I think my answer should be valid even for your original question: it is not something done routinely.


text:
How much does it cost to lease a jetliner?
As the title says, how much would it cost to lease a airplane for an airline. Let's say a B737 and a B747 or similar aircraft.
I tried to contact several aircraft leasing companies but so far I've been ignored, and searching this site hasn't resulted in any answers either.

If your second step is coming here, then there is probably a good reason that actual leasing companies ignored your question. This isn't quite "if you have to ask, you can't afford it", but it's close.

I am just interested in how much it is, I really have no intention of actually leasing a B747. There are many more questions about costs of such aircraft that receive answers. So unless its off topic it fits this site and could be a nice addition to it

Leasing costs are calculated in various ways. For example, short term leases might be priced per hour of flight time, with or without fuel (wet or dry), plus any additional expenses such as catering, landing/takeoff fees, customs fees, etc.
In other cases the calculation might be per seat/per mile. So, say you want to lease an A380 for 450 people on a 10,000 mile trip, you might be charged two US dollars per mile per person, and your party for 450 friends would run you a cool $9 million plus any additional charges, such as flight crew, food, fuel if it's a dry lease, etc.
Long term leases are a different story. There are many factors that play into that calculation starting from the aircraft type and year, your intended use of the aircraft, on which routes it will be operated, to the likelihood that you may default on the lease, and anything in between. That's why you were ignored; for airline leases they probably realized very quickly that engaging you in a discussion about that would waste their time.


text:
Why is the extra allowance for wheel installation on A380 in terms of degrees instead of torque/torque percentage?
When installing a wheel on the A380, the nut is permitted to be torqued an extra X degrees if the holes do not align.
Any idea why its in degrees instead of X torque or percentage of torque value?

For bolts, or threaded fittings in general, the torque varies widely with the lubrication. Perhaps that's the reason behind using repeatable references, such as that angle...

I'd be interested in a reference to where you question comes from. What is meant by "if the holes do not align"? Which nut? There's only one that holds the wheel on??? I'm not doubting you, I'd just like to get more info.

I'm guessing the OP may be referring to the holes used for lock wire.

Presumably the designers of the nut would know precisely how much torque an extra X degrees would apply. When you're trying to align holes, knowing the number of degrees you can turn the nut will be much more useful than knowing how many more N-m you can apply.

its a hole which fits a bolt. akin to a castellated nut and pin.

The reason is simple, the measurement from the point correct torque is achieved to the location of the hole for the bolt is in degrees, not ft-lbs (the number of degrees is absolute, whereas the force needed to get the alignment can vary). The torque specification has a built-in tolerance for additional torque needed for alignment. If you have to go more than X degrees to align it means you'll be applying excessive torque.


text:
Why don't nonrigid airships have multiple gas cells?
Nonrigid airships (blimps) differ from rigids and most semirigids in having the entire envelope form a single large gas chamber, rather than dividing the lifting gas among several redundant gas cells.  Having only one compartment for the entirety of the airship's lifting gas raises the specter of an envelope rupture causing the loss of the airship's entire gas load,1 resulting in ungood things happening to the airship and its occupants.
In contrast, having multiple discrete gas cells (either by using a number of separate internal gasbags contained within the outer envelope, or else by dividing the envelope itself into compartments using gastight septae), in the fashion of all rigids and almost all semirigids, would allow the airship to remain airborne even if one or two2 of the gas cells were ruptured and completely deflated,3 and would allow gas cells of uncertain structural integrity to be filled only partially (to reduce the stress on the cell walls, and, thus, the likelihood of a complete rupture) while still maintaining full inflation on the known-good cells.  In addition to the obvious safety benefits, the use of multiple gas cells would also allow better control of the envelope's shape (by allowing different gas cells to be inflated to different pressures to - for instance - stiffen the parts of the envelope experiencing the greatest loads), and would allow minor to moderate maintenance and repair work to be done on the envelope and/or individual gas cells without having to deflate the entire airship.
So why don't nonrigids have multiple discrete gas cells, rather than a single, undivided envelope?

1: This would be exacerbated by a nonrigid airship's need to maintain a considerable positive internal gauge pressure in order to prevent the aerodynamic forces on the airship from distorting its envelope; this positive internal pressure would greatly accelerate the escape of gas from the envelope, and the tension forces on the envelope resulting from said positive internal pressure would tend to produce severe tearing emanating from the site of any rupture (think of what happens when you pop a party balloon).
2: Or, potentially, more, depending on the size of the airship and the number of gas cells contained therein.
3: Note that this does not necessarily mean that the airship will remain capable of forward flight (although, depending on the circumstances [primarily which gas cell(s) lose pressure, and where within the envelope they happen to be located], it very well may), only that it won't be in danger of plummeting to the ground in the event of a rupture.  If a gas cell near the middle of the airship were to lose pressure, this would likely cause the envelope to distort to such a degree as to render controlled forward flight impossible, as @A.I.Breveleri correctly points out - but the key is that the airship would still have one or more intact gas cells providing a considerable amount of residual lift, allowing a controlled forced landing.4
4: This would be an especially great advantage in the case of a major rupture occurring at altitude (where the undivided envelope of a typical nonrigid could easily lose its entire gas load during the time needed to descend to ground level), and even more so if the temperature gradient of the ambient air is greater-than-adiabatic (in which case you lose lift as you descend even if you aren't progressively losing gas).

With the loss of one cell you also lose the same fraction of lift. Unless you can drop an equal amount of ballast quickly, having some more gas cells isn't much of an advantage – the ship still falls out of the sky.
The Zeppelins could lift up to three times their empty weight simply because they became more efficient by growing larger. Blimps are small, so their lifting capacity is restricted. Carrying so much ballast would require a much bigger blimp.

Non-rigid airships are structurally supported by the above-ambient air pressure inside the envelope.
It is difficult to have a non-rigid airship airworthy with one cell punctured and emptied and the rest intact. If a cell at the edge is punctured, the rest can maintain some structure. If it's a cell in the middle, everything to the outside of it will collapse. Either way, the remainder of the blimp will only be good enough to gently land for repairs.
Gently landing for repairs is something punctured airships are already able to do - the gas doesn't escape instantly. And as an airship is getting down, the gas inside expands (provided the air ballast bags are emptied), refilling the volume for a while longer.
So, this design decision would add weight, but not any extra capability or safety.
The only way to keep the airship structurally sound with some of its cells punctured is to add some sort of rigid structure - which would make it a rigid or at least a semi-rigid.

It is not at all clear how the partition between adjacent cells would be constructed, or what shape it would assume if one abutting cell lost pressure. The complete distortion of the airship's shape would be as much a disaster as the partial loss of lift.

One other possibility: the loss of one cell while the remaining cells were inflated, especially losing one in the center, would likely deform the entire airbag to such a state that the rest could be torn open by the stress.
In which case, a minor leak could become loss of all buoyancy, very quickly.


text:
What about the DC-9's wing makes wingletting it so difficult?
A common way of making older airliners more efficient without increasing their wingspan is to add winglets, either as a retrofit for existing aircraft or as a new, wingletted derivative of a preexisting non-wingletted model.
One particular older airliner for which this doesn't work, however, is the DC-9; winglets for the DC-9 have been studied, but never made it into production.  One of the main reasons for this (alongside the relative smallness of the benefit that could be gained by wingletting a DC-9, compared to that gained by wingletting other types of airliner, and the increasing smallness of the world's DC-9-classic1 fleet) is that the DC-9's wing structure makes it unusually difficult to safely and successfully winglet:

APB has investigated a retrofit design for the DC-9 family. Those design studies have not been successful in creating a viable business case. Projected block fuel burn reductions of less than 2 percent are offset by substantial modification costs. The limited potential for the DC-9 is a result of the existing wing structure, which hinders installation of a large winglet, as was possible on the Boeing 737 family. Since the DC-9 has been out of production since the early 1980s, the fleet size has shrunk and the fleet has aged, making the business case for a retrofit winglet or wingtip not as attractive as that for the Boeing 737 and 757 families.  [Emphasis added.]

What is it about the DC-9's wing that makes it so hard to winglet, compared to (say) a 737?

1: The DC-9-10 through DC-9-50, but not the DC-9-80 or DC-9-90.

Winglets are considered weaker than simply extending the wing, which they can't do because it won't fit in airport gates anymore.  Surely that's not a problem for the small DC-9; why not just extend its wingtips?


text:
FAF: Lightning Bolt vs Maltese Cross on approach plates?
Here is my understanding (please correct me if I am wrong):
A maltese cross is a FAF indicated with a distance and lightning bolt is FAF indicated by interception of glide slope. Non precision approaches like LOC, LNAV FAF is indicated by Maltese cross and ILS FAF is indicated by lightning cross.
However, what isn't clear to me is whether LPV and LNAV/VNAV approaches should use glideslope intercept or distance to indicate FAF?  Is this a DA vs MDA question?

The final approach fix on both LNAV/VNAV and LPV approaches is at glideslope intercept. The Maltese cross is only for non-precision approaches like LNAV or LOC.

LPV and LNAV/VNAV are officially "Approaches with Vertical Guidance" because they don't meet one (completely irrelevant) detail in ICAO's definition of "Precision Approach".
However, they have a DA, so for all practical purposes, you treat them as if they were Precision--and that means using the lightning bolt rather than the Maltese cross.

This is the answer I was looking for. If it has a DA, then lightning bolt. MDA is Maltese cross.

To expand on Chris' answer:
The Maltese cross designates the non-precision final approach fix. It may be a DME fix, intersection fix, or waypoint. The important point is that this is an identifiable fix, not just a distance.
The lightning bolt designates the glideslope/glidepath (ILS/MLS/TLS approach or RNAV, respectively) intercept point. On conventional procedures (i.e., those with a glideslope), the lightning bolt does not have to be colocated with a fix/waypoint. The actual glideslope altitude will be indicated in a smaller type size breaking the vertical component line.
On an RNAV procedure with vertical guidance, the lightning bolt will be located at a waypoint. This serves as the precision final approach fix (with "precision" meaning the approach has vertical guidance, not the ICAO definition of a precision approach). These approaches also use the smaller type size breaking the vertical component line. However, this value on a US TERPS procedure will be the same as the restrictive crossing altitude for that same waypoint.
The distant between components of the procedure is indicated in the baseline and is given to the nearest tenth of a nautical mile. This information is for advisory use only.


text:
How can we determine the maximum camber and thickness for airplane wings?
I want to know if there are any standard factors for determining maximum wing camber and thickness for heavy and light airplanes?
I'm using an optimization process to find the best wing shape to reach the best aerodynamics. E.g. in cruise mode to find the maximum lift-over-drag ratio, maybe choosing a thinner camber or thickness has a higher CL over CD ratio, but may lead to problems with structural limits.
Can anyone help me with some good references on the appropriate limits of changes in both maximum thickness and camber?

Chamber is good, but camber is not bed either.

@mins If you take the effort to make that comment, you might as well take the effort to click 'edit' and improve the post. It is a lot more helpful to a new user.

@DeltaLima: It was actually a joke (chamber/bed). Keep cool.

Camber pushes the lift coefficient operating range up and increases the pitching moment of an airfoil. It is advised to use lots of camber if you need to create lots of lift at low speed and have no need to fly fast. Conversely, high speed aircraft use uncambered wings. Flaps add camber - this makes them so effective for lowering the minimum speed of heavy aircraft.

Comparison of the same baseline airfoil at different camber values (picture source). This is taken from NACA Report 824 which later was turned into one of the most important books for aerodynamicists.
Wing thickness helps to build a lighter wing but will increase friction drag due to higher supervelocities caused by the displacement effect of the thick wing and cause earlier flow separation because the boundary layer is more stressed. In the end, you need to find a compromise: A 15% - 16% thickness is quite normal for GA aircraft, and high aspect ratio wings like those of the B-24 and the B-29 had a root thickness of  22%, but tapered the thickness down towards the outer wing as soon as practical. The tip thickness of both was just 9%.
The maximum lift will peak around 12% to 15% relative thickness. Below I copied figure 85 from NACA Report 460 to illustrate the point:

Now this was all subsonic aerodynamics. Once you move closer to the speed of sound, the supervelocities can cause pockets of supersonic air which end in a local shock. Therefore, the optimum thickness is reduced as the cruise speed approaches Mach 1, and wing sweep is added to cheat a little and keep thickness higher. Modern airliners use airfoils of around 13% relative thickness, and supersonic wings rarely use more than 6% relative thickness at the root, tapering to 2% to 3% at the tip.


text:
Has a Russian Tu-95 ever had to make an emergency landing in Europe?
Has a Tu-95 Bear bomber ever made an emergency landing in a European country while the flight was on a mission to test the country's defences?

Hi! Welcome to Aviation.SE. Unfortunately your question will probably be closed as too broad if you don't narrow it down

@kepler22b Not sure how this could be any more specific?

@JayCarr he could ask with some details like --which countries of Europe?,Europe is big--  When? For example from 1960-1970-- etc. :)

@kepler22b I would agree if it were something that were likely a common occurance (like general aircraft accidents).  But a Tu-95 crashing anywhere in Europe at any time (let alone while testing radar defenses) would be a significant event.  So no further detail is really required.  It's like the U-2 spy plane incident, for example.  You don't really have to specify a place or time because shooting down the U-2 was such a singular event, no further details are required beyond "Has a U-2 ever been shot down."

@JayCarr k,  let's see what the answer says...

the reason I asked is that with the huge distance these aircraft fly(now even close to the usa) and the age of the aircraft,it seems pure luck that they have never had a tech problem where they would have to put down in nato countries if unable to return to Russia or the old soviet union.

You do realize that Russia is in Europe right ?

If we think about the Cold War, I wouldn't think they'd even be able to enter the airspace of, say, a NATO country.  If they weren't over those countries in the first place, why would they land in them?  You could imagine a situation where an emergency arises over international waters, but you might then expect their orders would be to ditch at sea rather than try to land in an unfriendly country.

@Antzi Really?  By which defintion?  I think the question is clear.

@Simon most of the population lives west of the Ural mountain range, which lies in what is usually assumed to be Europe. It's a but iffy with Europe and Asia being mostly the same continent though.

I think the clue is in my question! I do realise that they do not enter other countries airspace(if they did,and were shot down,then I would place a bet that the start of a conflict could be predicted)but my question was,while flying over international waters,and a tech emergency took place,has permission ever been given in a mayday situation been given to land in a nato country.and yes I do know Russia has part of it in Europe as I live in Europe,and know these things!

A possible hostile bomber trying to make a emergency landing in a airfield strategicaly near their border... seens legit on the other hand outcomes can range from it being shot down to a nuclear war

Not that anyone knows of. A couple of Bears have crashed within Russia's borders.
They usually fly over international waters only, so if one did go down there would be a chance it would sink without a trace.

Sink. Or be sunk, just to be sure.


text:
Where to find US airport weather Service Levels?
Where can I find an airport's level of service for weather services?
In other words, how do I know if a given airport's Service Standard is a categorized as Service Level A, B, C and D?

For background, the FAA has determined Service Standards to categorize the levels of detail in weather observation sites. See the following from Appendix D of FAA JO 7900.5C (p. 152):

The term Service Standards refers to four levels of detail in weather observations at sites where there is a commissioned ASOS/AWSS. The first category, known as Service Level D, is completely automated service in which the ASOS/AWSS observation constitutes the entire observation, that is, no additional weather information is added by a human observer.
The second category, known as Service Level C, consists of all the elements of Service Level D, in addition to a human observer, who adds information to the automated observation. This is referred to as "augmentation." [...] Service Level C is provided at all airports with a properly sited, fully qualified Federal facility during facility hours of operation. During hours that the facility is closed, the airport reverts to stand-alone ASOS or Service Level D as described above.
The highest category, referred to as Service Level A, includes major aviation hubs and high traffic volume airports with average or worse weather. The remaining group of airports (smaller hubs or special airports in other ways, that have worse than average bad weather operations for thunderstorms and/or
freezing/frozen precipitation, and/or that are remote airports) are referred to as Service Level B airports

Welcome to aviation.SE! I've never heard of those levels, can you add some more information about them, and which country/region/regulator you're asking about?

Are "Service Levels A thru D"  FAA designations?

For some info describing the FAA Service Level designations, see FAA JO 7900.5C, specifically Appendix D, starting on p. 152.

The pilot's resource, Aviation Weather Services, AC-00-45, 3.1.3 says, "Whereas manual stations were most common prior to the mid-1990s in the United States, the vast majority of today’s METARs and SPECIs are from fully-automated stations."  @JonathanWalters Good find!

There used to be some correlation between airports with a control zone (cz) and towered airports having "manned" weather reporting.  The manned stations with control zones also provided "airport advisory service" monitoring traffic on 123.0.  You'd provide your position information to the weather station person, calling out "radio, N12345 is 10 miles out..." instead of calling "traffic" in the blind.  These are replaced with automated stations, and where there used to be control zones, Class E now extends to the surface in a keyhole shape protecting approach zones.

The last of the service standards issued by the FAA was a 2014 report and ONLY available via a FOI request.
According to AJT-21, the yearly updates have been discontinued (without giving any reason that we are aware of).  Last "published" report was 2013.
If you wish to see it, please email @ wxobservers@yahoo.com


text:
How is a light plane's nose wheel straightened during retract?
Assuming a light aircraft has direct cable linkage to the rudders, during taking off in a very strong crosswind with the pilot correcting with hard rudder, what keeps the nose wheel from remaining off center and jamming as it retracts?

Please add image source/copyright info


text:
What is that device on an Su-27 pilot's helmet?
While watching a YouTube video (2.5hrs), I noticed this device on the top of most Russian Su-27 combat pilot's helmet. A google search only shows it for a Su-27 and MiG-29, but apparently no other aircraft. It was not on any Chinese, Syria, Ukraine or Russian aerobatic pilots flying the Su-27. It appears to be at the wrong angle and has a metal plate that seems to exclude it from being some type of illumination.
What is this on a Su-27 helmet?

Possibly a night vision goggle bracket?

Looks like a go-pro mount.

Unrelated, but thank you for the documentary link. Truly interesting.

Russian Su-27 combat fighter aircraft designed around 1980. It started out as a lemon but the Russians improved it and variants so quickly over a few years that it soon out flew any other aircraft in the world. The F22 and F35 have been built specifically as a deterrent to the S-27 & Su-35. Russia challenged the US to a test dogfight. The US pilots did not like the idea...much, too risky to hold the fight...in front of photographers and TV cameras.
http://www.pravdareport.com/world/americas/20-07-2009/108253-su27-0/

@jwzumwalt we sure that's not an urban legend/ pravda piece?

That is the mounting point for night vision goggles. The following image shows a helmet with NVG mounted.

Helmet mounted NVG; image from lemt.by
The USAF has helmets with different mounting and some of the modern pilot helmets (like BAE Striker II, for example) have an integrated night vision device.

(spetstechnoexport.com)
Those are also mounts for Helmet Mounted Sight for the Vympel R-73 missile. Both the MiG-29 and Su-27 were equipped with this missile system.
I think F-35 pilots have this technology now but with a fancier helmet. The old ones on the MIGs looks like monocle.

@ymb SURA helmet-mounted target designator is the Su-30-adapted export version of "Щель-3УМ-1" system, which is used on MiG-29 and Su-27 (and its derivatives). It is indeed can be mounted on that slot on the helmet.


text:
What is the average weight difference between composite and metal airliners?
Is there any rule of thumb for weight savings using composite materials. For example, is there an early model large airliner like the B747 or even B737 that later was modified with composite wings or fuselage that shows weight savings?

composite is just as diverse as alloy. if you say alloy it could be iron-carbon aka iron or steel or aluminium-magnesium aka light alloy. composite is basically A (some kind of fiber, for tensile strength) reinforced B (some kind of solid, for compressive strength) ranging from rebar reinforced concrete aka concrete to fiber glass reinforced epoxy aka fiber glass or carbon fiber reinforced polymer aka carbon fiber. This is just like "flavor difference between gain powder and flour pasta" e.g. if you use the right thing do it right it's great or if wrong it's awful...

With about 50% of composites, Boeing and Airbus with the A350 and B787 claim to have gained more than 20% of the empty mass, but I have no reference. The use of composite materials is associated with other improvements, so it will be difficult to answer.


text:
AWACS - what do these antennas do?
In a press release for a new Boeing "wingman" (BATS) drone being built in Australia, a closeup of this Australian AWACS flying in formation was provided. What would be some of the specific uses for the antenna farm located all over the fuselage?

The image below labels each of the antennas. To summarize based on your groupings:
A: Multi-Role Electronically Scanned Array (MESA) made by Northrup Grumman. It has a similar role to the large rotating dome on the E-3 in tracking airborne targets. Instead of being rotated mechanically, it can be electronically scanned.
B: Communications/Navigation, GPS, ELT (emergency locator transmitter)
C: Missile warning, infrared countermeasure
D: Electronic surveillance
E: Communications
F: Communications
G: Communications/Navigation
H: Missile warning, electronic surveillance

Source

The large flat antenna along the spine replaces the rotating disks used on older Airborne Radar planes (e.g. AWACS), which physically point at each direction while transmitting and watching for the radar return, and therefore must be spun 360° to get a full picture.
It is an "Electronically Scanned" radar that electronically "steers" the radar beam. In this case, the left and right sides can scan 120°, while the bit on top of the wedge can scan the front and rear 60°, giving a 360° picture.
https://en.wikipedia.org/wiki/Active_electronically_scanned_array
https://en.wikipedia.org/wiki/Boeing_737_AEW%26C
Airborne electronic warfare systems incorporate a number of different features/modes. These can include searching for nearby (within a few hundred nm) aircraft, scanning for targets on the ground or sea, listening to transmissions by the opposing side, coordinating communications for your side, and possibly even active jamming to inhibit enemy weapons and communications. Different types of antennas will be used for specific frequencies.

Note that during the development phase of the MESA at Northrop Grumman's Baltimore facility, it was called Wedgetail internally.